i got a new 27" iMac 5k on thursday...i moved all of my data over. I shoot in RAW format which produces DNG files to work with in Lightroom and Photoshop. On my new iMac, every preview image (after hitting the spacebar) has a line or a square of a lighter section through it. To clarify, this is not via the actual Preview application...it's what happens when you hit the spacebar while on an image. If I move the image to my Dell external monitor right beside it the issue goes away. I pulled up the same files on my Macbook Pro (where they were) and no issue there either. Opening the image up in Lightroom is fine...no issues with the actual image. Here is a screenshot from my iMac and same image after being moved to my Dell monitor. What in the world?